1. Field of the Invention
The present invention relates to a surface-mount type multilayer inductor used as multilayer inductors, particularly as choke coils, LC filters, and other suitable inductors.
2. Description of the Related Art
A conventional inductor is disclosed in Japanese Unexamined Patent Application Publication No. 5-41324. The inductor is provided with a columnar magnetic core made of an insulating magnetic material such as ferrite. A conductor film is provided on the surface of the magnetic core, and then, the conductor film is irradiated with a laser beam and the laser beam moves in an axial direction while the magnetic core is rotated, a spiral coil-forming groove is formed and a coil spirally surrounding the magnetic core is defined by the remaining portion of the conductor film. In this way, a conventional inductor is made of a one-layer coil.
In the conventional inductors, such means as 1) using a magnetic core having a large sectional area, 2) increasing the number of turns of the coil, and 3) using a magnetic material having a high magnetic permeability as a magnetic core material are generally employed to increase inductance. However, the magnetic permeability of the magnetic cores and their dimensions (sectional area, length) are restricted and it is difficult to obtain the desired inductance. Moreover, when the number of turns of the coil is increased by reducing the width of a coil conductor to obtain the desired inductance, the DC resistance of the coil increases, and further the Q value of the coil decreases.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a compact multilayer inductor in which a high inductance is achieved.
A multilayer inductor according to the present invention includes a core member, a plurality of thin-film coils spirally wound and laminated on the surface of the core member, and terminal electrodes provided at the individual end portions of the core member, wherein the winding directions of the adjacent thin-film coils, having insulating layers therebetween, are opposite to each other, and wherein the plurality of the thin-film coils are electrically connected in series.
Preferred embodiments of the present invention provide separating portions for electrically connecting the thin-film coils in series, the separating portions arranged between an area where the thin-film coils are provided and the areas where the terminal electrodes are provided to surround the periphery of the core member, wherein the adjacent thin-film coils having the insulating layers therebetween are electrically connected in series through an opening portion for connecting the thin-film coils provided in the insulating layers.
The core member is, for example, dumbbell-shaped. Further, preferred embodiments of the present invention provide an identification portion for identifying the direction of the core member on at least one of an end face and a side face of the core member. Furthermore, at least one of the beginning portion and the end portion of a coil including the plurality of thin-film coils electrically connected in series is electrically connected to the terminal electrode through lead-out opening portions provided in the insulating layers.
When constructed as described above, the winding directions of the adjacent thin-film coils, having insulating layers therebetween, are opposite to each other, each individual thin-film coil of the plurality of thin-film coils generates a magnetic field in the same direction and the coils define one coil. In this way, the length of the core member is greatly decreased and the number of turns of the thin-film coils is increased as compared with an inductor where the plurality of thin-film coils is arranged side by side in the axial direction of the core member. Moreover, because a plurality of thin-film coils, having insulating layers therebetween, is arranged on the core member having a common axis, distributed capacitance is produced uniformly between the thin-film coils.
Moreover, the multilayer inductor according to the present invention includes second separating portions for forming separated areas electrically disconnected from the thin-film coils below the terminal electrodes, the second separating portions arranged between the area where the thin-film coils are provided and the areas where the terminal electrodes are provided to surround the periphery of the core member.
Based on the above construction, as the separated areas and the coil are electrically disconnected, even if layers are short-circuited below the terminal electrodes, a portion of the coils is not short-circuited and accordingly the coil construction is not adversely affected.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.